A lamp using a light emitting diode (LED) as a light source offers a number of advantages over conventional lamps such as an incandescent lamp, a halogen lamp and a fluorescent lamp. These advantages include longer lifetime and higher power-light conversion efficiency, as well as environmental friendliness in that mercury is absent in a LED lamp. Due to these advantages, LEDs have found many applications in lighting.
LEDs producing red, yellow, green or blue light are commonly available. These LEDs can be used in various applications such as in signpost displays and in traffic lights. In lighting applications used for illumination, e.g., illuminating a newspaper for reading, white light is preferred. Since white light has a broad range of wavelengths, and since light produced by a LED typically has a narrow range, an additional step is required to convert light produced by the LED into white light. A typical method is to employ a blue LED and a layer of phosphor that emits yellow fluorescent light when excited by a portion of the blue light produced by the blue LED. The mixing of the yellow fluorescent light and the remaining portion of the blue light yields the white light. However, there is often difficulty in mixing these two light components. Since the blue light produced by the blue LED is directional in general but the yellow fluorescent light is omnidirectional, there can be difficulty in mixing these two light components. It contributes to a result that when the white light is projected onto a white plane, the illuminated area appears to have an objectionable yellowish annular ring encircling the whiter area located at the center of the illuminated area. It results in perceivable non-uniformity of the color of the light emanating from the LED lighting device over the illuminated area.
The color of light is often measured in terms of “color temperature” The color temperature of the electromagnetic radiation emitted from an ideal black body is defined as its surface temperature in Kelvins, or alternatively in mired (micro-reciprocal Kelvins). Color temperature is used to compare light sources. However, since many light sources such as fluorescent sources and LEDs do not emit light by thermal radiation, the expression “correlated color temperature” is typically used to describe the “color temperature” for these light sources. It is based on human perception of the approximated “thermally radiated” color temperature. However, for convenience, the term “color temperature” as used herein refers to both actual color temperature and correlated color temperature.
The distribution of the color temperature can be made more uniform by means of complicated secondary optics, e.g., a textured reflector that is specially designed. While the manufacturing cost to produce a lighting device employing complicated secondary optics is increased, another disadvantage is that the lighting device cannot be miniaturized due to the presence of complicated secondary optics that are generally much larger than the LED. A lighting device that can be realized in small size without complicated secondary optics and that provides an illuminated area with perceived uniformity in color temperature is useful in many applications. Herein in the specification and in the appended claims, perceived uniformity in color temperature refers to a condition that an ordinary person perceives or subjectively feels that the color temperature (over an illuminated area) is sufficiently uniform. Although this can be quantitatively measured, lighting manufacturers sometimes rely on subjective observer evaluation. Further, as LED lighting fixtures are often designed to replace incandescent lighting fixtures, manufacturers often desire to replicate the emission of an incandescent bulb in creating a light source that is pleasing to consumers.
U.S. Pat. No. 7,973,327, discloses an arrangement in which a scattering ring is positioned to surround lateral surfaces of a LED such that a portion of light emitted from the LED through these lateral surfaces is scattered into a phosphor layer located on both the scattering ring and the LED. It is thought that due to scattering, the blue light emitted from the LED via lateral surfaces becomes much less directional when entering the phosphor layer, thereby promoting mixing between the yellow fluorescent light and the blue scattered light that is survived without being converted into fluorescent light. However, the inventors of the present invention determined that while the perceived uniformity in color temperature of the illuminated area was improved, the optical loss as a result of the addition of the aforesaid arrangement was as high as 10% to 20%.
In U.S. Pat. No. 6,917,057, it is mentioned that the spatial distribution of phosphor particles in a phosphor layer atop a LED is not uniform due to a step change in height at the periphery of the LED, causing uneven deposition of the phosphor particles over the phosphor layer during fabrication. It is disclosed in U.S. Pat. No. 6,917,057 that an arrangement of a substantially transparent, substantially phosphor-free layer of height at least one quarter of a height of the LED can first be deposited to surround the LED, so that the step change in height is reduced. The phosphor layer can then be deposited on top of the LED and the substantially transparent, substantially phosphor-free layer. However, it was determined that whereas the optical loss due to including the aforesaid arrangement could be maintained to be less than 7%, the non-uniformity of color temperature was unacceptably perceivable to viewers.
U.S. Pat. No. 7,999,283 discloses a light emitting device having an encapsulant with light scattering materials therein, and with scattering features to tailor the spatial emission pattern and color temperature uniformity of output light. The scattering features are tailored by spatially varying the concentration of light scattering materials. Although it is expected that the optical loss can be kept in a satisfactory value and the uniformity of color temperature can be made perceptually acceptable, a disadvantage is the fabrication complexity involved in the deposition of the encapsulant, thus leading to a high manufacturing cost.
There is a need in the art for a lighting device that provides an illumination with perceived color uniformity in the light emanating from the device without leading to a substantial optical loss, and that can be realized in small size and without involving substantial fabrication complexity.